The center frequency stability of a wideband FM modulator utilizing a voltage controlled oscillator (VCO) is inversely proportional to the maximum frequency deviation of the modulator. An automatic frequency control (AFC) loop can be incorporated to provide increased stability of the VCO frequency. AFC loops can be categorized as primarily analog or digital in nature. The standard analog AFC loop designs become non-linear in applications requiring large deviations and wide bandwidths. The standard digital AFC loop designs have highly linear transfer characteristics but possess inherent problems which limit the frequency stability improvement.
The major elements of the AFC loop are: a frequency discriminator capable of linearly converting the modulator frequency deviations to an error voltage; and, a loop filter which must adequately filter the error voltage such that the modulating signal does not appear at the VCO input. The discriminator is a frequency to voltage converter. As the input frequency to the discriminator changes, a corresponding voltage change occurs at the discriminator output. The VCO input is composed of the desired modulation signal (generated by a signal conditioner or the like) and an error signal (generated by the discriminator). Obviously, the discriminator and signal conditioner cannot possess the same frequency response because the error voltage would tend to cancel the desired modulation. The discriminator must be capable of detecting very slow center frequency drift rates, in the presence of FM and provide enough error signal to correct for the drift. Thus, the discriminator's frequency response must be of a low pass characteristic. Since the desired modulation and the error signal cannot occupy the same frequency domain, the first two system restrictions are imposed. One, the desired modulation signal must possess a high pass characteristic, and two, the error signal must possess a low pass characteristic.
Considering the nature of FM, the presence of a center frequency spectral component may be non-existent. It will be necessary for the discriminator to convert the entire output spectrum of the VCO to a corresponding voltage at the output of the discriminator and not rely on the presence of a single frequency component. Not only must the discriminator possess a broad band response at its input but it must possess a high degree of symmetry about its origin at the output. Traditional discriminators possess the familiar "S" curve transfer function. A high degree of origin symmetry and linearity is maintained over only a small portion of their dynamic range.
Existing designs of discriminators employ pulse (time averaging) techniques or tuned circuits. The time averaging techniques is the most linear of the two schemes for FM demodulation, and possesses a wider input bandwidth than the tuned circuit. A commonly used time averaging discriminator is a monostable multivibrator of a known pulse width and externally triggered on each new cycle of input frequency. Time averaging discriminators are discussed at page 419, 420 of Operational Amplifiers, published by Burr-Brown, copyrighted 1971, and edited by Jerald G. Groeme, Gene E. Toby and Lawrence P. Huelsman. Monostable multivibrators are not well known for constant pulse width over temperature. Thus, while these prior art discriminators may provide the required linearity and bandwidth, they are extremely unstable over wide ambient temperature fluctuations and over long terms.